1. Field of the Invention
This invention relates to flexible pipe joints and more particularly to flow-through swivel pipe joints adapted to perform in an underwater high pressure environment.
2. Description of the Prior Art
Undersea pipe laying operations are important to a number of activities, particularly in the petroleum exploration and production industries. Many offshore oil wells, for example, require a line to be laid from the wellhead to a point at which the production from the well is gathered. One method used to bury a pipeline in a seabed involves towing a specially designed sled along the seabed by means of a tow line attached to a barge positioned on the surface of the water. The sled is positioned around and follows a pipeline which has previously been laid on the seabed. Pumps on the barge supply high pressure water to jetting nozzles mounted on the sled. The nozzles are directed towards the seabed so that the high pressure water stream digs a trench as the sled is towed along the seabed. The pipeline is progressively laid into the trench behind the sled as the trenching operation proceeds across the seabed. One example of such a pipe burying apparatus is disclosed in U.S. Pat. No. 4,041,717.
A high pressure water line is used to convey water from the barge pumps to the jet nozzles on the sled. Because a large amount of relative motion is normally experienced between the sled on the seabed and the barge on the surface of the water, some means of imparting flexibility to the high pressure connection between the barge and the sled should be provided in the system. The water lines themselves may be constructed of flexible material. In addition, it has been found desirable to incorporate a flexible pipe joint into the system to connect the high pressure line to the jet nozzles at the sled. This pipe joint typically includes an inlet passage, which is connected to the high pressure water line from the barge, and two outlet passages, each of which is connected to a conduit through which the high pressure water is conveyed to the jet nozzles.
The performance requirements of such a joint are demanding. It must be tightly sealed so that the high pressure water flowing through the joint may not escape, but the joint must be relatively friction free so as to retain flexibility under severe conditions without experiencing an abnormally short usuable life span. The durability of such a joint is especially important because of the remote undersea location in which it is used, with the consequently high cost of replacing such a joint. These requirements are further complicated by the corrosive salt water environment in which such joints normally are operated.
Flexible joint designs which are adapted to operate under high internal pressure are known in the art. Some such designs utilize a spherical ball and socket type design to give the joint the desired freedom of movement. Such joints typically have pressure balancing devices and are preloaded by using some means to apply opposing forces which clamp the joint around the spherical ball element. The complicated design of such joints, however, makes them unsatisfactory when used in an undersea pipe burying apparatus of the type described above. Such joints typically exhibit an excessive amount of friction in motion, resulting in an abnormally short life span.
Other high pressure joint designs utilize ball bearings in the joint to counteract the adverse effects of a high amount of friction. It has been found, however, that the use of such joints introduces an excessive cost into the pipe burying system. The two outlet passages for such joints are in axial alignment and require that the fixed outlet conduits on the jet sled, to which the joint is connected, also be in axial alignment within close tolerances. It has been found that maintaining these close tolerances during construction of the jet sled is difficult and costly.
Consequently, there is a need for a simple, reliable, and economical flow-through swivel pipe joint design which may be used in high pressure applications.